There are known film cryotrons of the type that comprises a Josephson junction. These are superocnducting switching devices wherein the critical current of the distributed Josephson junction is controlled by a magnetic field acting upon said junction. A cryotron of this type comprises a superconducting ground plane and two superconducting film electrodes with supply lines. The ground plane and electrodes are arranged one above another and insulated from one another. The electrodes are interconnected by a elongated distributed Josephson junction, for example, a tunnel junction. The cryotron also includes at least one superconducting control line arranged above one of the electrodes and extending along the Josephson junction. The elongated distributed Josephson junction is a junction of which one of the linear dimensions is much greater than the depth of penetration of a magnetic field into the junction, whereas its other linear dimensions are either less than that depth or commensurable therewith. The current gain factor of such cryotrons is not more than two; the curve of their control characteristic, i.e., the curve representing the critical current of the fate versus the control current, is of a shape that differs considerably from the most desired rectangular shape.
The above disadvantages are eliminated in a cryotron of which one of the electrodes and its supply lines are of specially selected shapes. This electrode is a screened strip extending along and adjacent to the Josephson junction. A screened supply line comprising one or more parallel branches is connected to the middle of this strip. At least one control line is arranged above the strip and extends along the Josephson junction. At the points of connection of said supply line to the electrode strip, the width of the branches of the supply line is not in excess of that of the strip; these points of connection are spaced from the points of intersection of the strip boundaries and the control line at a distance which is greater than both the strip width and the depth of penetration of a magnetic field into the Josephson junction. This type of cryotron compares favorably with other known types based on the Josephson junction in that it has a higher current gain and in that its control characteristic curve is of a shape that is more convex and closer to the rectangular shape.
However, an improvement in the control characteristic is due to an increase in the geometrical dimensions of the cryotron which, in turn, is accounted for by two factors. First, the cryotron's area is increased by the area occupied by the branches of the supply line. Second, the control line is arranged above the electrode strip and extends alongside the electrode. This means that the width of the electrode strip cannot be less than the width of the control line plus the allowance for matching of the control line with the strip boundaries. As a result, the width of the electrode strip is 2 to 4 times greater than the minimum possible width. There is another important consideration that must be borne in mind. In order to have a control characteristic curve of a convex shape, it is necessary that the strip length be 5 to 8 times greater than the width of the strip, so that the actual minimum length of the strip is 10 to 30 times greater than the minimum possible length. The dimensions are increased to a still greater extent in the case of the multi-input cryotron which employs two or more control lines arranged side by side in one layer.
There is further known a film cryotron based upon the Josephson effect and having relatively small dimensions, despite the fact that it features a plurality of control lines (cf. IBM Technical Disclosure Bulletin, vol. 17, No. 11, April 1975, p. 3483, the article by B. S. Landman and H. H. Zappe).
In this type of cryotron, a hole is made in the ground plane opposite one of the electrodes which itself is provided with a hole partially overlapping the hole in the ground plane so that the electrode is shaped as a closed loop. One side of the loop is in contact with a distributed Josephson junction, whereas its opposite side is above said hole in the screen. A plurality of control lines are interposed between the screen and electrode, above the hole in the ground plane. Due to the hole in the ground plane, the inductive coupling between the control lines and the electrode loop is sufficiently strong and roughly equal for all the buses.
As a result, the current that flows through the control lines induces a current in the electrode loop and, consequently, in the Josephson junction, which brings the latter to a resistive state. The current gain of the cryotron under review is not more than two, and the shape of the control characteristic curve of this cryotron is far from rectangular. This is due to the fact that the shape of the electrodes does not provide for spatial separation of the transport current flowing through the Josephson junction and the current induced in that junction by the magnetic field produced by the current of the control lines.
It is known that the current gain of a cryotron with a plurality of control lines can be raised by connecting two or more control lines in series by means of superconducting strips. In this case the current through the control lines must flow in the same direction, and the current flowing through said strips must produce no parasitic magnetic field in the region of the Josephson junction. The above requirements account for the fact that the control lines and connecting strips intersect with each other, and this in turn, accounts for a more complicated design of the cryotron.